Handle null assetFd like a FNF

Bug: 140961740
Test: TODO

We already catch an FNF and attempt to open the file as an InputStream.
Do the same if null is returned.

I haven't figured out a way to make assetFd set to null and yet opening
an InputStream succeeds, so this is untested.

Change-Id: Iabd05db714bc693ead2dc8cc4c0b46fef9f33d5a